After an oil well has been partly drilled and suspected producing formations have been penetrated, it is necessary to make various tests to determine production possibilities of various formations. One of the test techniques involves the use of a tool which is known as a formation tester. One exemplary formation tester is set forth in U.S. Pat. No. 4,375,164 assigned to assignee of the present disclosure. As set forth in that disclosure, the tool is adapted to be lowered into the well bore, supported on an armored logging cable enclosing certain conductors for providing surface control for the tool. The logging cable extends to the surface and passes over a sheave and is spooled on a reel or drum. The conductors in it connect with suitable surface located power supplies, controls, and recorder. The formation tester is lowered to a specified depth in a well. At that elevation, a backup shoe is extended on one side of the formation tester and formation testing apparatus is extended diametrically opposite into the formation of interest. The equipment so extended normally includes a surrounding elastromeric sealing pad which encircles a smaller extendable snorkel which penetrates a formation as the formation will permit, up to a specified depth. The snorkel is ideally isolated from fluid and pressure in the well to be able to test the formation. The snorkel is extended into the formation to enable direct fluid communication from the formation into the tool. Moreover, it is isolated from invasion of the well borehole and pressures therein to permit a pressure sensor to obtain formation pressure. Further, a sampling chamber elsewhere in the formation tester can be selectively connected through the snorkel by suitable valves to obtain delivery of a fluid sample from the formation. The fluid sample typically may include a relatively small sample which is a pretest sample, and if that is acceptable, a larger sample can be drawn through the snorkel. Various pretest and sample volumes are selected and determined under control from the surface. As will be understood, the tool body is typically only a few inches in diameter (depending on hole size) and thus is not able to store substantial quantities of formation fluid. Thus, a sample is taken, the storage chambers therein filled, and the formation tester is retrieved.
Other measurements can be made including various and sundry tests for formation permeability. Ideally, such measurements are obtained isolated from the intrusion of the well borehole. One of the factors resulting from the intrusion of the well borehole is the drilling fluid which is routinely used to conduct well drilling operations. It is normally identified as drilling mud. In the well borehole, the mud forms a mud cake against the side wall of the drilled hole. This helps isolate the various formations. The drilling mud thus packs against the side wall and the liquid in the drilling mud may penetrate relatively deep into the adjacent formations while the solid particles in the drilling mud form a filtrate cake. This cake tends to be somewhat dry as a result of the loss of liquid therein by filtration into the adjacent formations.
When the formation test procedure is terminated, the equipment extended from the formation tester is retracted. Thus, the snorkel is pulled in and the seal around the elastomeric gasket is normally broken equalizing the pressures surrounding the pad. The backup shoes extended on the opposite side are also retracted. Typically, this procedure occurs with the formation tester (normally, a elongate cylindrical body) pressed against one side of the borehole formation. There is a possibility of pressure differential sticking of the sealing pad, or even the cylindrical body. A representative sticking problem is discribed in U.S. Pat. No. 3,724,540. There, differential pressure sticking is set forth as a retreival problem. Differential sticking arises from the circumstance wherein pressure in the well is greater than the formation pressure. When the sealing pad is pressed against the filtrate cake, the hydrostatic pressure of the well fluids in the borehole might be sufficiently greater to hold the sealing pad against the mud cake. Assume, for instance, that the pressure in the adjacent formation is somewhat less than the pressure in the borehole. If this occurs, the sealing pad and snorkel is held against the mud cake, even embedding into it, and it may be held so tightly that it cannot be retracted. The sticking problem may act on the sealing pad and tool body both. The full retraction of the sealing pad and snorkel may not break the sealing force; if so, the tool body is held against retreival. Even worse, the sticking may hold the entire tool body.
One way to dislodge the formation tester is to simply lift up on the logging cable which supports the formation tester. This, however, runs the risk of breaking the cable because the vertical lifting force required is extreme compared to the normal operating loads placed on the logging cable. The total surface area exposed to differential sticking can be substantial and accordingly, the axial load required to pull the tool free is quite great.
U.S. Pat. No. 3,724,540, is an apparatus for disengaging the formation tester from the borehole wall. By contract, this disclosure sets forth push-off pistons which are located above and below the sealing pad and snorkel to extend simultaneously towards the adjacent formation with a view of breaking differential sticking which holds the body against the formation. The push-off pistons are maintained at a normally retracted position. They incorporate piston rods extending laterally of the tool body which support large thrust pads for wide footing when they extend. The piston rods are connected with suitable pistons in the hydraulic cylinders within the tool body. The tool hydraulic system is utilized to provide hydraulic power for extending the push-off pistons whereby differential sticking is thus broken. This is particularly beneficial because it aids in overcoming the sticking force on snorkel retraction.
With this in view, the present invention is thus summarized as a formation tester having push-off pistons, preferably two with one located above and the other located below the snorkel, the push-off pistons being powered by the tool hydraulic system for the purpose of pushing the tool body away from the formation to thereby break the differential sticking.
Further objects and advantages of the present disclosure will become more relatively apparent upon consideration of the discription of the preferred embodiment in conjunction of the drawings described below.